Hydrocarbyl substituted sulfonates are widely used as emulsifiers for many purposes, including the formulation of emulsifiable lubricating compositions for use in metalworking processes such as metal cutting, grinding, and milling. For many years, natural sulfonates have been used in such applications. Natural sulfonates are relatively inexpensive materials that are by-products of refining processes, however, such natural sulfonates often have wide supply availability fluctuations and have disadvantages with respect to quality and emulsifying properties. In an attempt to improve the emulsifying properties of natural sulfonates, it is customary to associate such natural sulfonates with secondary surface active agents of different types, more especially non-ionic types or fatty acid salts. However, in order to achieve consistent results, the amount of these secondary compounds used differs according to the quality of the natural sulfonate in the composition. Accordingly, there continues to be a need for sulfonates suitable for metalworking applications having improved properties.
With regard to the foregoing, an exemplary embodiment of the disclosure provides an alkali metal sulfonate mixture suitable for metalworking applications. The mixture includes a reaction product of a first synthetic mono-hydrocarbyl substituted sulfonic acid having from about 20 to about 50 carbon atoms in the hydrocarbyl substituent, a second synthetic mono-hydrocarbyl substituted sulfonic acid having from about 10 to about 16 carbon atoms in the hydrocarbyl substituent, and an inorganic alkali metal base. The resulting reaction product has a number average molecular weight ranging from about 400 to about 500 and a substantially bimodal molecular weight distribution.
In another exemplary embodiment, there is provided a method for making an alkali metal sulfonate mixture having improved properties. The method includes the step of charging a mixture of sulfonic acids to a reaction vessel. The mixture of sulfonic acids contains a first synthetic mono-hydrocarbyl substituted sulfonic acid having from about 20 to about 50 carbon atoms in the hydrocarbyl substituent, and a second synthetic mono-hydrocarbyl substituted sulfonic acid having from about 10 to about 16 carbon atoms in the hydrocarbyl substituent. The mixture of sulfonic acids is neutralized with an alkali metal inorganic base selected from the group consisting of sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate, potassium carbonate, and lithium carbonate to form a reaction product in an oleaginous reaction medium. Water is removed from the reaction product to provide the alkali metal sulfonate mixture having improved properties. The product mixture has a number average molecular weight ranging from about 400 to about 500 and a substantially bimodal molecular weight distribution.
An advantage of exemplary embodiments described herein is that the resulting synthetic sulfonate mixture has superior emulsibility properties and enhanced rust inhibition properties compared to conventional metal sulfonate products typically used for metalworking applications.